The present invention generally relates to detectors for the collection of photons and more particularly relates to non-steady-state photoinduced electromotive force (photo-EMF) detectors.
Non-steady-state photoinduced electromotive force (photo-EMF) devices can generate time-varying photocurrents in response to a corresponding lateral and rapid shift of an optical pattern across its surface, and rely on the formation of space-charge gratings in semi-insulating materials.
In a typical application, the optical pattern is the result of a set of optical fringes incident on the detector (generated by the interference of a pair of coherent beams), and its lateral shift is due to a rapid phase shift of one beam relative to the other. Transient photocurrents are detected in an external circuit when the differential phase modulation frequency on one or both beams exceeds the response rate of the space-charge gratings. The non-steady-state photo-electromotive-force was proposed and first experimentally demonstrated by Petrov et al. (Sov. Tech. Phys. Lett 12, 379 (1986))
Using this detection technique in a homodyne interferometer, Stepanov et al, showed that vibration amplitudes could be measured in the picometer range, which is in the range of surface displacement induced by laser-based ultrasound. (Opt. Lett. 15, 1239 (1990)).
This laser homodyne receiver is adaptive, removing the effects of speckle and compensating for low-frequency ( less than 10 kHz) environmental perturbations. One drawback of the photo-EMF detector is its relatively low homodyne detection responsitivity per radian of optical phase modulation (xcx9c10xe2x88x925 A/W-radian).
The low responsitivity of the photo-EMF detector is caused, in part, by the large electrode spacing relative to the drift length of the photocarriers. The relevant figure of merit that characterizes the responsivity is the photoconductive gain g, which can be defined as   g  =                    L        D            W        =          τ              τ        transit            
where LD is the drift length, W is the electrode spacing, T is the carrier lifetime and Ttransit is the carrier transit time across the electrode width W.
Higher responsitivity at constant incident optical power can be obtained by reducing the electrode spacing in transverse-contact devices and focusing the optical beams. However, focusing the highly speckled beams is not typically compatible with the small reference beam/signal beam crossing angles that are required for optimum grating period. Higher responsitivity values can also be obtained using longitudinal electrodes in thin broad-area devices using tilted gratings, but the high capacitance of this geometry, coupled with the need for grazing-angle addressing of the device, pose practical bandwidth and field-of-view limitations of this approach, respectively.
The present invention provides for transverse-field geometry photo-EMF devices that have improved responsitivity at constant power and detection area without the need to focus tightly, therefore, allowing the optimum beam crossing angle in laser-based ultrasound receiver applications. The devices use asymmetric interdigitated contacts (AIDC) with alternating wide and narrow active-area spacings, with the current s from the wide-spaced active-area regions summed, while the narrow areas are optically blocked or rendered insensitive.
A photo-EMF detector for the collection of photons generally includes a substrate formed from a semi-insulating semiconductor, with sufficient carrier trap density to form an effective space charge grating, along with a plurality of interlaced electrode pairs disposed over the substrate. Each electrode pair includes two parallel electrodes defining an active area therebetween for the collection of photons, with one electrode of each pair being disposed between an adjacent pair of electrodes and proximate one electrode of the adjacent pair.
Importantly, means are provided for preventing back action current between proximate electrodes. Such back action current is of opposite sign, or direction, and will oppose the desired output. The means for preventing back action current may comprise a means for preventing light from striking the substrate between the proximate electrodes or a means for desensitizing the substrate between the proximate electrode, as, for example, implanting ions into the substrate.
More particularly, the means for preventing light from striking the substrate may comprise a reflecting or absorbing layer extending between the proximate electrodes. Alternatively, back action current may be prevented by utilizing grooves in the substrate between proximate electrodes or for providing slots in the substrate between proximate electrodes. The groove and slots also provide a means for reducing capacitance between proximate electrodes.
Means are provided for collecting outputs from each of the plurality of interlaced electrode pairs. In essence, the current is collectively summed across every other electrode pair.
Importantly, each electrode pair includes two electrodes defining an active area therebetween which can be small enough (i.e., one active area region per fringe spacing) to collect current from every fringe of a set of optical fringes incident the detector. Accordingly, the photo-EMF detector in accordance with the present invention provides improved responsitivity for a constant detector area without the need to focus.
In order to provide in-phase current collection, means are provided for summing outputs is provided. In addition, in one embodiment of the present invention, a heterostructure may be disposed between the substrate and the plurality of interlaced electrode pairs.
In one embodiment of the present invention, a detector may be provided with a first and a second plurality of electrodes arranged with one another on a substrate as hereinabove described. The first and second plurality may be disposed on the substrate adjacent one another (i.e., side by side) on the substrate or in a stacked relationship on the substrate. In the side by side relationship and the stacked relationship, the plurality of electrodes may be perpendicular with one another. Alternatively, in the side by side relationship, the first and second plurality of electrodes may be parallel with one another.
In addition, areas between the side by side relationship, the substrate may be desensitized between the pluralities of electrodes.